Editing IPCC:AR6/WGI/Chapter-5 (section)

==== 5.3.2.1 Reconstructed Centennial Ocean Acidification Trends ====

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Ocean pH time series are based on the reconstruction of coral boron isotope ratios ( δ <sup>11</sup> B). A majority of coral δ <sup>11</sup> B data have been generated from the western Pacific region with a few records from the Atlantic Ocean. Biweekly resolution paleo-pH records show monsoonal variation of about 0.5 pH unit in the South China Sea ( [[#Liu--2014|Liu et al., 2014]] ). Interannual ocean pH variability in the range of 0.07–0.16 pH unit characterizes southwest Pacific corals that are attributed to El Niño–Southern Oscillation (ENSO) ( [[#Wu--2018|]] [[#Wu--2018|H.C. Wu et al., 2018]] ) and river runoff ( [[#D’Olivo--2015|D’Olivo et al., 2015]] ). Decadal (10-, 22- and 48-year) ocean pH variations in the south-west Pacific have been linked to the Inter-decadal Pacific Oscillation, causing variations of up to 0.30 pH unit in the Great Barrier Reef ( [[#Pelejero--2005|Pelejero et al., 2005]] ; [[#Wei--2009|Wei et al., 2009]] ) but weaker (about 0.08 pH unit) in the open ocean ( [[#Wu--2018|]] [[#Wu--2018|H.C. Wu et al., 2018]] ). Decadal variations in the South China Sea pH changes of 0.10–0.20 have also been associated with the variation in the East Asian monsoon ( [[#Liu--2014|Liu et al., 2014]] ; [[#Wei--2015|Wei et al., 2015]] ), as a weakening of the Asian winter monsoon leads to sluggish water circulation within the reefs, building up localised CO <sub>2</sub> concentration in the water due to calcification and respiration.

Since the beginning of the industrial period in the mid-19th century, coral δ <sup>11</sup> B-derived ocean pH has decreased by 0.06–0.24 pH unit in the South China Sea ( [[#Liu--2014|Liu et al., 2014]] ; [[#Wei--2015|Wei et al., 2015]] ) and 0.12 pH unit in the south-west Pacific ( [[#Wu--2018|]] [[#Wu--2018|H.C. Wu et al., 2018]] ). Since the mid-20th century, a distinct feature of coral δ <sup>11</sup> B records relates to ocean acidification trends, albeit having a wide range of values: 0.12–0.40 pH unit in the Great Barrier Reef ( [[#Wei--2009|Wei et al., 2009]] ; [[#D’Olivo--2015|D’Olivo et al., 2015]] ), 0.05–0.08 pH unit in the north-west Pacific ( [[#Shinjo--2013|Shinjo et al., 2013]] ) and 0.04–0.09 pH unit in the Atlantic Ocean ( [[#Goodkin--2015|Goodkin et al., 2015]] ; [[#Fowell--2018|Fowell et al., 2018]] ). Concurrent coral carbon isotopic ( δ <sup>13</sup> C) measurements infer ocean uptake of anthropogenic CO <sub>2</sub> from the combustion of fossil fuel, based on the lower abundance of <sup>13</sup> C in fossil fuel carbon. Western Pacific coral records show depleted δ <sup>13</sup> C trends since the late 19th century that are more prominent since the mid-20th century ( ''high confidence'' ) (Pelejeroet al., 2005; [[#Wei--2009|Wei et al., 2009]] ; [[#Shinjo--2013|Shinjo et al., 2013]] ; [[#Liu--2014|Liu et al., 2014]] ; [[#Kubota--2017|Kubota et al., 2017]] ; [[#Wu--2018|]] [[#Wu--2018|H.C. Wu et al., 2018]] ).

Overall, many of the records show a highly variable seawater pH underlaying strong imprints of internal climate variability ( ''high confidence'' ) and, in most instances, superimposed on a decreasing δ <sup>11</sup> B trend that is indicative of anthropogenic ocean acidification in recent decades ( ''medium confidence'' ). The robustness of seawater pH reconstructions is currently limited by the uncertainty on the calibration of The δ <sup>11</sup> B proxy in different tropical coral species.

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